Advice on changing MTU needed, due to VPN constantly disconnection

[rmhrc]

I have searched many threads on here, and many seem to suggest changing the MTU. Here is my scenario:

Basically I am running 2 VPN's to 2 remote offices, over SDSL, and they're connection keeps dropping (eg Outlook constantly loses its connection). Also, when I try to remote desktop to one of their pc's, I only get about 30 seconds out of it, then lose the connection. Surfing the internet on both sides is absolutely fine, so I'm guessing its nothing to do with our SDSL itself.

Each of the 3 sites has a D-Link firewall, of which allow you to change the MTU. I have ascertained that our optimum is 1424, using TCPoptimizer.

My first question is: Is it enough to just change the MTU's on each firewall, or do you need to do it on the remote client's pc too?

I have tried changing the MTU to 1424 on all firewalls, and also on my own pc (using a registry change I found on here), but having tried another remote desktop connection, the problem is still happening.

My second question is: Is it worth me trying to lower it any further, and if so, how low should you realistically go? I have heard 576 mentioned.

And thirdly, could it be something else? Any help would be much appreciated.

 

[msworld]

For VPN, you can try to setup 1400. this page may help,

How to change mtu
... use a fixed maximum transmission unit (MTU) size of 1500 bytes for all PPP
connections and use a fixed MTU size of 1400 bytes for all VPN connections. ...

http://www.howtonetworking.com/VPN/mtu1.htm

Bob Lin, MS-MVP, MCSE & CNE
How to Setup Windows, Network, VPN & Remote Access on

http://www.HowToNetworking.com

 

[rmhrc]

Should I be setting the MTU on the clients as well as the firewalls?

 

[technome]

Personally I would not use a default fixed MTU at the router
or in the registry setting, especially in the registry, as this affect the internal network, fixed is for a Windows RAS VPN setup.
Each broadband line can have packet losses from routers along the path, each individual line can be different.

Use the TCP Optimizer tool below, discover the optimal MTU size to set at the router, then pathping from the office to the remotes public address or from the remote to the office public address, if the MTU is set correctly you should have few if any lost packets. For an explanation, search "Black hole router". You might try the pathping command first, to see how many lost packets you are now getting

http://support.microsoft.com/default.aspx?scid=kb;en-us;q314825

http://www.speedguide.net/downloads.php

........................................
Chernobyl disaster..a must see pictorial

http://www.kiddofspeed.com/default.htm